1. Field of the Invention
The present invention generally relates to multimedia data communication and, more particularly, to a method and apparatus for synchronizing packetized multimedia data in and among a plurality of receivers.
2. Description of the Prior Art
Multimedia data includes audio, video, and video plus audio data, which may be delivered by wired or wireless communication devices and methods. Examples of wired communication include Ethernet, other high speed networks such as HomePlug®, or other power line network systems. Wireless communication examples generally include RF or infrared data links. Regardless of the type of medium, multimedia signals are typically converted to digitized form for transmission because of the advantages in efficiency and spectrum utilization over the traditional analog forms of modulation and transmission.
In a conventional digital system, an original analog signal is sampled by an analog-to-digital (ADC) converter to produce discrete digital data which may be processed in a variety of ways and compressed to reduce its bandwidth and improve the transmission rate required. In such systems the data may be grouped into data packets of a predetermined size and content (packetization) before being transmitted through the selected medium. Data packets may be transmitted point-to-point, or point-to-multipoint including broadcast mode (to all receivers) or multicast mode (to a specified range of receivers). Of all the receivers in the transmission medium set to receive signals produced by the transmitter in the medium, only those receivers having the address included in the packet header will be able to receive the data packets addressed to them and to obtain the data contained therein. The packets also contain information, such as a CRC checksum, to verify the received data. Operations are carried out in the receiver to decompress and reassemble the data, and convert it back to analog form for playback. The data packets are generally transmitted at constant intervals depending on the sampling rate, compression ratio, packet size, etc. Ideally, the packets arrive periodically and “in step” at the receiver for reconstruction as a continuous stream of video or audio.
However, in the real world, with normal transmission media, the loss or corruption of data packets is inevitable because of the effects of noise and interference. As is well known, electrical power lines provide a medium subject to wide variations in transmission conditions, including high levels of noise and interference. It is then necessary to retransmit lost packets, which delays the arrival of the packet at the receiver, or skip the lost packets, which results in gaps in the perceived playback. Further, when the transmitter and each of the receivers operate independently of each other in time, encoding and decoding the data at their own clock rate, processing operations occur asynchronously. Individual time differences may accumulate, resulting in noticeable artifacts such as echoes, garbled or lost data, or unintended noise, sounds or blemishes during playback. In several important examples, if the transmitter encodes data faster than the receiver decodes the data, the packets are generated at a higher rate than they are consumed. Periodically, the excess packets may accumulate and be discarded, resulting in a loss of some of the data. Similarly, if the transmitter encodes data slower than the receiver decodes the data, the packets are generated at a lower rate than they are consumed. Upon playback, the continuity of the data may be subject to being interrupted intermittently, resulting in gaps in the sound or video image. Moreover, in the case of distribution of multimedia data from one source to multiple receivers, echos may occur in the playback of data from receivers that are not playing the data at the same pace, producing unacceptable artifacts or omissions. These problems may be especially troublesome when the receivers are within audible or visual proximity of each other.
Several methods have been proposed and are in use to provide for synchronizing the signals received by a plurality of receivers to correct for these deficiencies. For example, in conventional systems, extra data packets, called “start” packets may be transmitted periodically along with the packetized data to provide a time reference for the receiver to maintain the operation of its decoder at the same rate as the encoder in the transmitter. Alternatively, the transmitter may broadcast a replica of its clock signal to all receivers to provide a synchronizing signal for all of the receivers to “lock on” to the same clock that controls the encoder. Both of these methods, while effective, consume valuable bandwidth, an important resource in a wideband multimedia signal. Further, such synchronization methods can result in unintended collision events.
Succinctly stated, the problem presented by the prior art is how to synchronize a plurality of receivers receiving a multimedia signal when several receivers are within audible or visual proximity to each other, and how to keep the playback of the multimedia program at each receiver location synchronized with the transmitter, without the use of additional clock signals, which consume bandwidth and/or result in increased collision events. What is needed is a method of synchronizing multimedia signals in the receivers in a simpler, more efficient way when transmitted by a single transmitter.